The present invention relates to a method for the catalytic reforming of naphtha with a high yield of BTX aromatics. More particularly, the present invention relates to a method whereby a high yield of BTX aromatics is obtained from a full-boiling-range naphtha feed stock without the need for any pretreatment steps such as splitting the naphtha into a low boiling and a high boiling stream. The catalytic reforming of naphtha is an important refining process in the petroleum refining industry with the goal of producing the following substances:
1. High-octane hydrocarbons for the gasoline pool or PA1 2. BTX-aromatic compounds (benzene, toluene, xylene) as important chemical raw materials or solvents.
In the known process of catalytic reforming, naphtha is reacted on a catalyst at temperatures between 300.degree. and 600.degree. C. under hydrogen gas at pressures of 1 to 50 bars. Under these conditions, different reactions occur in parallel to each other such as isomerization, dealkylation, dehydrocyclization, dehydrogenation, hydrocracking and hydrogenolysis. Which processes predominate is a function of the reaction conditions, of the composition of the particular naphtha stream and of the catalyst.
Gasoline represents a mixture of many hydrocarbons. An octane number can be associated with each hydrocarbon. A high octane number is desired in gasoline for use as fuel for internal combustion (Otto) engines. High-octane hydrocarbons are isoparaffins and aromatics.
The proportion of aromatics and especially of benzene in gasoline has been constantly reduced in recent years on account of their toxicity in order to meet environmental requirements. It can be expected that the permitted aromatics content in gasoline will be further reduced in the coming years.
On the other hand, BTX aromatics are important raw materials for the chemical industry. For example, styrene based plastics, phenolic polymers, nylon and biologically degradable detergents are produced from benzene. Toluene is used as a solvent in dyes; ortho- and para-xylene are processed to polyester fibers, plastic foils and plastic bottles. Metaxylol is normally isomerized catalytically to para-xylene. Thus, reforming processes with the highest possible yields of BTX aromatics are desired for these applications. Various extraction methods are known for separating the BTX aromatics from the non-converted aliphatic hydrocarbons; see for example, Kirk-Othmer "Encyclopedia of Chemical Technology"; 3rd edition, vol. 9, pp. 707-709.
Catalytic reforming is carried out in various system types. All methods comprise several series-connected reactors in which the catalyst is located. In the so-called semi-regenerative and cyclic systems the catalyst is built into the reactors in the form of a fixed bed. In the modern CCR systems (CCR: "Continuous Catalyst Regeneration") the catalyst, consisting of small spheres, flows slowly through the reactors and is discharged after a certain dwell time (2 to 7 days).
The catalyst is slowly covered during the operation with residues (coke) containing carbon. This brings about a noticeable deactivation.
The deactivation speed is a function of the type of naphtha used, the process conditions and the catalyst. After a certain coke content is deposited on the catalyst it is no longer profitable to continue to use the catalyst. It must be regenerated in order to restore the activity. In semi-regenerative and in cyclic systems the regeneration is carried out in the reactor and in a CCR system this takes place externally. After regeneration the catalyst is subsequently reintroduced into the reactor. The frequency of regeneration in the CCR method is distinctly higher in comparison to the two other methods.
The state of the art in the area of catalytic reforming has achieved a high level in the meantime as a result of years of research activities. Due to the large amounts of naphtha which are to be matched to their later usage by means of reforming processes, even rather small advances in this art constitute a large economic advantage.
Catalytic reforming has been carried out since the 70's with preference being given to the use of bimetallic catalysts. The catalysts generally contain a platinum group metal (preferably platinum) and a second metal (usually designated as a promotor) which improves the catalytic qualities of the catalyst. Aluminum oxide is used as catalyst carrier. Furthermore, a halogen component is present, usually chlorine.
Rhenium is frequently used as the second metal of the catalyst. Platinum-rhenium catalysts are distinguished by a very good stability.
In recent years Pt--Sn catalysts have been increasingly used. They yield a distinctly higher liquid yield than the Pt--Re catalysts at the same octane number. The lower stability of the Pt--Sn catalysts in comparison to Pt--Re catalysts must be compensated for by a higher frequency of regeneration. Pt--Sn catalysts are therefore preferably used in CCR systems since in these systems the time between two successive regenerations is relatively short in comparison to other systems. The more stable Pt--Re catalysts are preferably used in semi-regenerative and cyclic systems as fixed-bed catalysts.
The boiling range of the naphtha to be reformed plays an important part in the optimizing of the BTX yield. The precursors of benzene have a boiling point between 71.degree. and 82.degree. C. and the toluene precursors between 82.degree. and 121.degree. C. (D. M. Little in "Catalytic Reforming", Penn Well Publishing Company, 1985, chapter 6). The xylene precursors (C.sub.8) boil between 106.degree. and 130.degree. C.
On the other hand, fractions of naphtha with a boiling range between 65.degree. and 175.degree. C. or between 71.degree. and 171.degree. C. are used in the production of BTX aromatics. Even more narrow boiling ranges are used such as e.g. 84.degree. to 157.degree. C. In every instance naphtha with these boiling ranges also always contains C.sub.9+ hydrocarbons, since the boiling ranges of C.sub.8 hydrocarbons and of C.sub.9+ hydrocarbons overlap to a large extent (Kirk-Othmer "Encyclopedia for Chemical Technology"; 3rd edition, vol. 4, pp. 264-277). The reformate of these naphtha fractions therefore generally contains considerable amounts of C.sub.9+ aromatics in addition to the BTX aromatics.
Elevated yields of BTX aromatics can be achieved by means of engineering measures. European patent EP 0,234,837 describes a reforming method for a high-boiling naphtha fraction (boiling range 118.degree. to 198.degree. C.) with which elevated BTX yields can be achieved using two different catalysts in two zones. A Pt--Sn catalyst is used in the first zone and in the second zone a platinum catalyst which can additionally contain Re. Since two catalysts must be used in this patent this method can not be used in the CCR systems corresponding to the newest state of the art, which generally contain only one regeneration system.
U.S. Pat. No. 3,943,050 also describes a 2-stage method for reforming naphtha fractions of the gasoline boiling range of approximately 50.degree. to 215.degree. C. for the production of high-octane blended components for motor fuels. A catalyst without zirconium is used in the first stage. A zirconium-containing catalyst is used in the second stage. The reformate formed is characterized by octane number only as regards its use in gasoline. The patent furnishes no indications about the level of BTX amounts in the reformate.
German patent DE 29 20 741 describes a method for depositing zirconium on an oxidic carrier. In that process, zirconium is deposited onto the carrier from a zirconium-containing solution containing at least one zirconium complex ion which is formed from an organic acid, preferably oxalic acid. Correspondingly formed catalysts (which also contain Cl, Pt and Sn in addition to Zr) are used in the examples in the isomerization of C.sub.8 aromatics, of paraffins and in the reforming of n-heptane. An elevated yield of BTX aromatics was not established.
A catalyst is described in German patent DE 24 55 375 which contains at least one of the metals zirconium, titanium or tungsten in addition to a platinum group metal and tin and which is used in the reforming of hydrocarbons. In the aromatization of C.sub.6 to C.sub.7 hydrocarbon mixtures with a Pt--Sn--Zr catalyst the selectivity as regards the aromatics is between 45.8 and 53.8%, depending on the composition of the feed.
U.S. Pat. No. 4,197,188 describes catalysts which contain rhenium in addition to a platinum group metal and zirconium. French patent 2,187,885 describes a reforming catalyst containing at least one of the lanthanide metals in addition to aluminum oxide, platinum and zirconium. The reformate obtained is characterized by octane number only as regards its application for gasoline. No individual substances (such as e.g. benzene, toluene or xylene) are considered.
In addition to the above-mentioned applications for reforming, Zr-containing catalysts are used in isomerization (e.g. DE 26 39 747, DE 26 15 066 and DE 27 31 669).
Prior art methods have also included splitting the naphtha feed into a low boiling fraction and a high boiling fraction; see U.S. Pat. No. 4,401,554.